1. Field of the Invention
This invention relates generally to a tendon tension sensor for measuring the tension on a tendon using conduit reaction forces on a conduit through which the tendon is threaded and, more particularly, to a tendon tension sensor for measuring the tension on a tendon in a robotic hand, where the sensor is mounted to an end of a conduit that the tendon is threaded through and where the sensor measures conduit reaction forces on the conduit.
2. Discussion of the Related Art
Dexterous robot systems are known in the art that perform a variety of functions. A dexterous robot system typically includes a robotic arm having a robotic hand with fingers and related joints that operate to grasp an object or part for a particular application. In one dexterous robot system design, tendons are employed to actuate the fingers where the tendons are coupled to the finger joints. The actuators that actuate the tendons to move the fingers are typically positioned within the forearm area of the robotic arm. Tendons extend from the actuators to the finger joints, where they are attached. Typically, it takes two tendons to actuate a single joint of the finger, one tendon to close the finger and another tendon to open the finger.
The tendons are sometimes threaded through a conduit that isolates the joint actuation from configuration changes occurring between the actuator and the joint. The conduit applies reaction forces to its support structure that serves to counter the tendon forces. This keeps the actuator joint forces from interfering with upstream joints in response to torque provided by the joint. Further, the conduit maintains the length of the tendon constant. Thus, if the wrist of the robotic arm moves, the conduit maintains the length of the tendon constant so that the fingers do not move.
To close the loop on a robotic finger force control, feedback is needed for the tendon tension. Directly sensing the tendon tension has proven to be challenging. Several factors contribute to this challenge including providing a location downstream of a major friction force, where space in the downstream robotic assembly, typically a robotic palm, is severally limited. Further, the tendon is not stationary.
One known technique for measuring tension in a robotic tendon employs a strain gauge sensor for measuring the tensile deformation of the tendon. However, the range of tensile forces that are encountered in robotic applications may be so small that strain gauge sensors may not be sensitive enough to provide an accurate measurement. Also, the strain gauge sensor doesn't measure the deformation of the tendon, but employs a strain element to introduce a bend in the tendon so that a greater tendon tension results in a greater strain in the element. However, there is typically not enough space for such a sensor.
It is also known in the art to use load cells to measure tension. However, commercially available load cells typically are too large for robotic applications in that they cannot be adequately housed inside of a robotic arm.
Another known design employs an S-shaped elastic element on which a strain gauge is mounted. An end of a tendon is coupled to one end of the S-shaped element and an end of another tendon is coupled to an opposite end of the S-shaped element so that tension on the tendon causes the S-shaped element to deform. This design requires cutting the tendon and the element has a relatively large diameter. This design also presents a problem of lack of space as well as having wires hanging in space attached to a moving piece.